Using multiple, wired, physical monitors simultaneously is a common practice among computer users. There are several solutions available today that provide this using hardware present on the computer system itself. There are also several software solutions that make a virtual display within the computer system and make the virtual display available to a monitor or other device connected to the computer in ways other than with standard video cables. Current software solutions are hampered by the limitations of Windows 7 and later operating systems in making virtual displays available to the user.
Windows Vista introduced a new driver model that was tied very closely to hardware. As such, it was not intuitive to developers how a virtual version of this driver model could be accomplished. According to requirements set by Microsoft, a display adapter (also called a “physical display device”, a “video card”, and a “graphics card”) has to appear as though it was both physically attached to the computer system and had physical resources for use. If a device driver is written without appearing as though it is tied to physical hardware in the computer system, the operating system will consider the driver invalid and refuse to proceed.
Solutions adequate for previous versions of Windows employed a well-documented mechanism by which a mirror display driver projected a mirror display beyond the bounds of the existing display. Due to an accidental or intentional design consideration by Microsoft, this appeared as a new physical display to versions of Windows older than Windows 7.
Since Windows Vista, however, the newly proposed architecture for display drivers, known as the Windows Display Driver Model (WDDM) architecture superseded the old architecture, known as the XP Driver Model (XPDM). Since mirror drivers were strictly XPDM in design, they are now obsolete on machines running Windows 7 and newer Microsoft operating systems. For Windows 8, there is no mechanism by which XPDM display drivers can continue to be used. Without mirror drivers, there is no obvious way to virtually extend a desktop with Windows 8 and potentially any future Microsoft operating system releases.
It's important to note that even though Windows 7 is compatible with XPDM display drivers, the computer system will suffer noticeable performance degradation while such drivers are in use. When used with adequate hardware, Windows Vista and later versions use an accelerated desktop mode known as Aero. This desktop mode enhances the look-and-feel of the display, both aesthetically and in responsiveness to the user. Aero was pitched as one of the enhanced features of Windows Vista and many users expect it as part of their experience. Unfortunately, with an active XPDM display driver, Windows is forced to disable Aero to provide a compatibility mode between the physical display device and the enumerated virtual one. One consequence for some applications while in this mode, beyond a changed aesthetic, is reduced rendering performance.
Another solution that has been presented is installing a filter driver over a functioning physical display adapter within the computer system. This filter driver is transparent to the physical display device and simply modifies the physical display device's behavior slightly so as to force an additional display on the physical display device.
In a sense, this filtering method works by tricking the physical display adapter into sensing that another physical monitor has been connected to the computer system. The filter driver then intercepts this output and renders it to an alternate target. This method does not force Windows into a compatibility mode like XPDM drivers do, and it is generally successful in making an additional display available to the user. This solution is less than ideal, however, as it requires intimate knowledge of drivers made by specific hardware manufacturers. The risk is high that when a manufacturer releases an update of their display drivers, subtle changes may break compatibility with this filtering method. Moreover, this method is exceptionally incompatible with Windows 8 and later versions, and may not be possible at all.
Another drawback to this filtering approach is that since it forces display detection in physical hardware, previously existing hardware can present problems with activating a virtual display. This filtering method is generally incompatible with computer systems where multiple monitors are already in use. Additionally, this method is also limited by the physical hardware as to how many actual virtual monitors may be iterated on the physical display device.